This invention relates generally to the field of radar, and more particularly to systems for tracking radar targets.
Conventional tracking radars are designed on the tacit assumption that there is only one target present within one or two beam widths of angular spread. Consider a phased-array radar observing two independent targets at the same range. Assume that the targets are not resolvable in the range or Doppler dimensions and that they are spaced close enough so that a beam oriented in the direction of one of them will pick up appreciable energy from the other. Conventional tracking radars are unable to make accurate measurements on either target under these conditions.
The problem of estimating the angle of arrival of two closely spaced targets has received considerable attention. An investigation of the maximum likelihood estimate (MLE) of the angles of arrival is presented in the article "Low Angle Tracking in the Presence of Multipath", by W. D. White, IEEE Trans. on Aerospace and Electronics Systems, Vol. AES-10, No. 6, Nov. 1974, pp. 835-852 which is hereby incorporated by reference. White calculated the mean-square elevation error for the two target case when the signal-to-noise ratio (SNR) was large and hence the error was small. This calculation was made for the symmetric case (target and image are symmetrically located about the center of the elevation antenna pattern) and for the nonsymmetric case. The symmetric case occurs when the antenna is stabilized and hence the symmetry plane of the elevation antenna pattern coincides with the flat reflecting plane.
There are several problems associated with the maximum likelihood estimates for the individual elements of an array. First, the resulting equations are difficult to solve and require search techniques. Furthermore, it is difficult to ensure that the global or absolute minimum is obtained because search techniques operate on the principle of finding points of zero slope and may terminate at one of those points which is not the absolute minimum. Second, a large number of receivers is required for element processing, which may make a practical system costly.